Most of the tin plate used for can manufacture is electrolytic plated. It is generally appreciated by persons skilled in the art that tin plate surfaces are usually treated by what is commonly termed "passivation".
One type of passivation, known as 300 involves dipping the tinplate in hot chromic acid and chromic salts, producing chromium oxide and tin oxide on the tin plate surface. A second type of passivation, known as 311, involves cathodic treatment of the tinplate in a dichromate solution maintained at a positive potential. The tin plate which is maintained at a negative potential receives a coating of metallic chromium, chromium oxide and tin oxide.
A variation of the 311 passivation treatment, known as 314, requires a dichromate bath maintained at an elevated temperature and a higher current than the 311 treatment and produces relatively more metallic chromium. A further passivation treatment, known as 320, involves anodic treatment in dichromate and has been found to be unsatisfactory from a lacquer adhesion standpoint.
All of the known passivation treatments, of which 311 is the most popular, do not produce predictable or uniform results. Thus two different batches of tinplate, both of which having undergone 311 passivation may exhibit drastically different lacquer adhesion qualities. Tin plate treated by the 320 passivation treatment normally exhibits the least satisfactory lacquer adhesion qualities.